A security systems are known for locating a user upon activation of a signal transmitter. For example, U.S. Pat. No. 5,223,816, to Levinson et al., discloses a system that can only detect a large zone within which the user, who activated a transmitter, is located. A long time searching in this zone is then needed to find the user. This system is thus neither fast nor accurate. U.S. Pat. No. 5,223,844, to Mansell et al., discloses a system for vehicle tracking that uses a global positioning satellite signal. However, the system must depend on the global position system and is only applicable to vehicles.